This invention relates to jam detection in a printer and, more particularly, to detecting carriage jams by sensing current in a carriage drive stepper motor.
Detecting paper jams in a printer is important for several reasons. First, continued application of driving force to the carriage drive motor after a paper jam may cause physical damage to the print head, its associated mounting structure, or other carriage hardware. Also, paper may be bunched or compacted to the point where its removal is difficult without disassembling the print head or another part of the carriage structure. An improperly detected jam may result in the destruction of a payment document, such as the only personal check a customer may have. In addition, data sent to the printer for printing may be discarded and possibly irretrievably lost if the printer electronics are not notified on a timely basis that the data has not been successfully printed.
Traditionally, paper jams near the print head carriage in a printer have been detected using some combination of interruptive sensors (i.e., see-through sensors) and comb strips. These implementations require additional mechanical hardware as well as extra electrical circuitry. Extra hardware in the carriage area not only adds cost and takes up valuable space within the printer, but it may contribute to paper jams in the carriage area by its very presence. Typical jam detection hardware, monitoring electronics and software are taught in U.S. Pat. No. 5,074,690 for PRINT HEAD CARRIAGE HOMING SYSTEM, issued Dec. 24, 1991 to James R. Del Signore, II, et al.
Many small, low-cost printers such as those used in point-of-sale (POS) checkout stations use stepper motors to drive their carriages. Typically, only a subset of the multiple stepper motor windings are driven simultaneously. This allows current induced in other, undriven windings to be monitored and the instantaneous status of the stepper motor movement deduced. Monitoring electrical current provided to a motor, whether stepper, DC, AC, etc., has long been used as a technique to deduce the motional status of the motor.
U.S. Pat. No. 4,383,209 for CONTROL SYSTEM FOR TRANSDUCER POSITIONING MOTOR, issued May 10, 1983 to Martyn A. Lewis, teaches a control system for a stepper motor in which both voltage and current are sensed in a closed-loop controller. The thrust of the LEWIS circuit, however, is to enhance the positional accuracy of a transducer such as a read head in a disk drive. Both a constant voltage and a constant current source are provided with seamless switching, while coarse seeking the constant current source provided the motor drive. During fine seeking of the track position, the constant voltage source provides the motor drive. LEWIS, however, teaches no monitoring of either current or voltage to determine a stalled condition such as would be experienced during a carriage jam in a printer.
The system of the present invention monitors phase current to derive a logic signal indicating to the printer controller that a carriage jam has occurred.
U.S. Pat. No. 5,032,781 for METHOD AND CIRCUIT FOR OPERATING A STEPPER MOTOR, issued Jul. 16, 1991 to Klaus Kronenberg, teaches another stepper motor controller wherein an induced voltage in a winding, which is not required at the time to produce a propulsive force, is evaluated and a stopped motor condition detected. KRONENBERG stores the instantaneous position of the motor in memory for later recovery.
In contradistinction, the inventive system monitors the current waveform of a driven winding of the carriage stepper motor. An amplifier and a comparator are used to digitize the analog voltage waveform from across a sense resistor in the stepper motor""s drive line. An operational amplifier having its gain optimized for the application is used to ensure that digitization of the current waveform corresponds accurately with known values in a digital count table.
U.S. Pat. No. 5,074,690 for PRINT HEAD CARRIAGE HOMING SYSTEM, issued Dec. 24, 1991, to James R. Del Signore II, et al., teaches using a ribbon timing strip and a sensor for printer jam detection. No current sensing in a stepper motor winding is disclosed.
Another stepper motor drive system is disclosed in U.S. Pat. No. 5,367,239 for PRINTER CARRIER DRIVING METHOD, issued Nov. 22, 1994 to Tsuyoshi Matsushita, et al. MATSUSHITA, et al., teaches a stepper motor drive where improved control of acceleration and deceleration is achieved from a single-voltage power source. Stored data corresponding to acceleration, constant speed printing, and deceleration are used to generate a current reference voltage. There is, however, no teaching of jam detection through monitoring of either current or voltage supplied to the stepper motor.
The inventive system, on the other hand, monitors current in a single winding of the stepper motor at the commutation time of another winding to derive highly accurate jam detection (i.e., stalled carriage) information.
U.S. Pat. No. 5,431,502 for CARRIAGE MOTOR CONTROLLER FOR PRINTER, issued Jul. 11, 1995 to Yasunori Orii, et al., teaches yet another stepper motor controller for a printer carriage. ORII, et al. use a rotary encoder to generate position data and generate acceleration/deceleration commands based upon both absolute position data from the encoder as well as from stored motor characteristic data. The stored motor characteristic data is periodically updated to reflect the current operating characteristics of the stepper motor. There is, however, no teaching of carriage jam detection through voltage and/or current monitoring.
The inventive system, on the other hand, develops a jam signal from the A to D conversion of a voltage signal obtained from a low value series resistor in one of the stepper motor winding drive lines. An operational amplifier having a carefully selected gain functions to provide a signal indicative of operational current levels that can be read by commercial A/D converters. The inventive system monitors the current waveshape in the xe2x80x9cPhase Axe2x80x9d winding at the time when the xe2x80x9cPhase Bxe2x80x9d commutation occurs. By selecting this time window, the accuracy of the detection is greatly enhanced.
U.S. Pat. No. 6,150,789 for STEPPER MOTOR CONTROL, issued Nov. 21, 2000 to Robert Pulford, Jr., teaches sensing current in one winding of a stepper motor for control purposes. PULFORD discloses no sampling of current specifically during the commutation of a different motor phase winding. Neither does PULFORD specifically teach jam detection in a printer carriage.
None of these patents taken individually or in any combination teaches or suggests the carriage jam detection method of the present invention.
It is therefore an object of the invention to provide a carriage jam detection method for use in a printer which requires neither a comb nor flag and sensor mechanism to reliably detect carriage jams.
It is a further object of the invention to provide a carriage jam detection method for use in a printer which monitors current in a single, active winding of a stepper motor driving the printer carriage.
It is an additional object of the invention to provide a carriage jam detection method for use in a printer which monitors current in a first stepper motor winding at approximately, but not limited to, the time that a second winding of the stepper motor commutates.
It is another object of the invention to provide a carriage jam detection method for use in a printer which does not necessarily require an additional, external motion sensor.
It is a further object of the invention to provide a carriage jam detection method for use in a printer which utilizes a small resistor in the motor drive circuit as a sensing resistor to obtain a voltage waveform representative of the instantaneous motor winding current.
The present invention is a method for monitoring the current waveshape in a first winding of a stepper motor at or near the time when a second winding of the stepper motor commutates. The current waveshape is received from a low-value sensing resistor, typically within an integrated circuit stepper motor driver device. Changes in the current waveshape during this time window may be interpreted and accurate deductions may be made concerning the loading of the stepper motor. In the case where the stepper motor is the drive motor for a printer carriage, the loading information may be used to detect a paper jam, similar problem, or reference position in the printer carriage. No external monitoring sensor is required, nor is there any need for traditional comb or similar look-through structures for use in combination with a light source and sensor for detecting carriage jams.